1. Field of the Invention
The present invention relates to telephony such as that using Voice over Internet Protocol (VoIP). In particular, the present invention relates to a call control method for falling back to a secondary network when there is insufficient quality on a primary network. Still more particularly, the present invention relates to a call control method for falling back from an IP network to a secondary network such as a public switched telephone network (PSTN).
2. Description of the Background Art
Long distance data communication became extremely popular with the acceptance of the Internet Protocols and the Internet. Commonly, the Internet was used for data consisting of email or files for web browsing, but not for voice telephone calls. As IP networks improved, telephone calls became an addition to Internet traffic. But there are problems using the Internet for telephone calls, as the IP technology breaks up a call into packets, which may have to compete with other data, unlike the PSTN, which uses a dedicated, circuit-switched network connection for a call. Once a telephone call is established, the PSTN provides calls with very understandable speech and with very high reliability. IP networks must have high enough bandwidth to pass calls, and in addition have either a priority mechanism to favor voice traffic or enough extra bandwidth to avoid losing or delaying packets, which degrade the quality and understandability of a voice call.
IP telephony is used as it is frequently lower cost than PSTN calls, and for businesses, equipment for IP telephony may be lower cost than previously used circuit-switched equipment. IP telephony equipment may also have lower maintenance cost than circuit-switched equipment. Advanced telephony features can be provided at no extra cost from an IP network, as features are just packets, whereas circuit-switched networks tend to charge extra for features.
An example of an advanced set of features (called advanced features from here on out) is an IP phone at a remote location from the main business. The IP phone uses the IP network for a range of advanced features such as transfer, conference and directory listing. The phone also uses the IP network for its graphical user interface (GUI). As the user looks up another user in the directory, the graphical user interface can display the presence information of the other user—for instance, whether the user is on the phone or out of the office. Features such as transfer and conference may be purchasable at extra cost from the PSTN, but more advanced features such as the graphical user interface may not be available at all, giving the IP phone an advantage over previous circuit-switched technology. For example, the GUI can also be used to indicate whether the user has voice mail, and allow the user to select and listen to a particular message. On low quality IP networks the message can be retrieved via file transfer and then played locally on the IP phone, providing high voice quality. Similarly, Instant Messaging (IM) and Text Messaging—sometimes implemented with Short Message Service (SMS)—are features that can be incorporated into an IP phone but are not offered on the PSTN.
Another advanced feature example is an auto attendant at a remote location from the main business. The remote location uses PSTN lines for local numbers, allowing customers to make low-cost local calls to contact the business. A customer calling the remote location is connected to an auto attendant offering destination options. By connecting the remote office to the main business with IP telephony, the customer can request an operator, and the operator can be located as the main business. The operator can take the call on the IP network, and then transfer the call to a user at the remote location using the IP network. The PSTN call remains connected to the remote office and requires no additional extra-cost features from the PSTN.
The prior art has attempted to provide fault tolerance to IP telephony networks by using PSTN such as described in U.S. Pat. No. 6,868,080 entitled “Voice over internet protocol call fallback for quality of service degradation”. For example, there are instances when the IP network is completely down, or unable to transmit IP call control. In this case to make a call, IP telephony is abandoned, and calls are placed entirely on the PSTN and have only PSTN functionality. The benefits of IP call control are lost in this instance.
In both of the above examples, the IP network can reliably provide the features required, but may have difficulty providing the quality needed for the media of the phone call. Thus, there is a need for a mechanism to improve the quality of calls made over an IP telephony network and retaining the advanced features offered by the IP telephony network.